The blood flow to the peripheral circulation is regulated to maintain a balance between the delivery of nutrients and the metabolic demands of the tissue. Blood flow increases to a metabolically active tissue. This increased blood flow is the result of an increase in arteriolar diameter of terminal arterioles and larger upstream vessels. The terminal arterioles appear to determine distribution of flow while the upstream or "feed" vessels are more important in regulating total tissue flow. With an increase in metabolism there is the release of vasoactive metabolites from the tissue. Although vasoactive metabolites are known to affect the diameters of terminal arterioles the mechanisms by which metabolic factors regulate the diameter of upstream arterioles is uncertain. Recent studies from our lab and others have indicated an important role for the venular-arteriolar diffusion of vasoactive substances. The studies outlined in this proposal will test the following working hypothesis. In response to mismatches in blood flow and tissue metabolism, decreases in PO2 and/or increases in PCO2 and H+ are directly or indirectly sensed by the venular endothelial cells, resulting in the release of vasoactive metabolites of arachidonic acid which regulate the tone of adjacent arterioles. The proposed studies will test this hypothesis utilizing recently developed in situ microcirculatory techniques in which we are able to selectively remove the venular endothelium and assess the impact on blood flow regulation. The proposed studies will test two specific hypotheses: 1) The venular endothelium releases a metabolite of arachidonic acid that diffuses from the venule to the arteriole to cause an arteriolar dilation in response to an increase in tissue metabolic rate. 2) The venular endothelium responds to hypoxia, hypercapnia, and acidosis, directly, or indirectly to initiate the release of one or more vasoactive factors from the venular endothelial cells. These studies should provide new and important information relevant to our understanding the importance of venular-arteriolar diffusion of endothelial derived factors in the regulation of blood flow.